1. Field of the Invention
The present invention relates to an optical element for imparting a phase difference (retardation) to the light transmitted therethrough and to a method for manufacturing same, and more particularly to an optical element to be incorporated in a semi-transmissive semi-reflective liquid crystal display device to provide a phase difference suitable for transmissive display or reflective display and to a method for manufacturing such optical element. Furthermore, the present invention also relates to a semi-transmissive semi-reflective liquid crystal display device incorporating such optical element.
2. Description of the Related Art
A liquid crystal display device has been suggested that combines the function of a reflective liquid crystal display device in which the light introduced into the liquid crystal display device from the outside (external light) is reflected by a reflection plate and passed through a liquid crystal layer, switching and displaying the bright-dark state of the screen (termed “reflective display”), and the function of a transmissive liquid crystal display device in which the light emitted from a light source that was installed in advance inside the liquid crystal display device passes through a liquid crystal layer and switches and displays the bright-dark state of the screen (termed “transmissive display”). In such semi-transmissive semi-reflective liquid crystal display device, transmissive display and reflective display are performed at the same time. When the environment is comparatively dark, mainly the transmissive display is viewed, and when the environment is bright, mainly the reflective display is viewed. Because such display devices can be used in a variety of stages, regardless of the ambient conditions, they have been widely employed in the field of mobile devices that are supposed to be used mainly outdoors in bright environment.
The following semi-transmissive semi-reflective liquid crystal display device can be considered. FIG. 7 shows an example of a semi-transmissive semi-reflective liquid crystal display device.
In a liquid crystal display device 100, a liquid crystal layer 103 is inserted between laminated members 151, 152 comprising glass substrates 101, 102, and light irradiation means 117 comprising a light source 122, a light guiding plate 123, and a light reflecting member 124 is disposed in a position on the outside of the outer side surface of the laminated member 152.
Reflecting plates 104a are disposed with a certain spacing at the inner side surface of the glass substrate 102, a protective layer 119 is provided between the inner side surface of the glass substrate and reflecting plates 104a, light transmitting sections 104b are formed between the adjacent reflecting plates 104a, and the light transmitting sections 104b and reflecting plates 104a constitute a semi-transmissive semi-reflective layer 104.
Here, in the laminated member 152, the region where the reflecting plates 104a are disposed corresponds to a reflective display region serving as a region for reflecting the light used when a bright-dark display is performed in the reflective display, and the region where the light transmitting sections are provided corresponds to a transmissive display region serving as a region that passes the light used when a bright-dart display is performed in the transmissive display. Furthermore, in the laminated member 151, on an interface with the liquid crystal layer 103, a reflective display region or transmissive display region is formed in a position facing, in the thickness direction of the laminated member 151, the light reflecting section 104a or light transmitting section 104b of the laminated member 152.
A transparent electrode 108 comprising a transparent electrically conductive film such as Indium Tin Oxide (hereinafter abbreviated to “ITO”) is laminated on the inner surface side of the glass substrate 101, and an oriented film 107 is formed so as to cover the transparent electrode 108. A ¼ wavelength plate 120 and a polarization plate 114 are provided on the outer surface side of the glass substrate 101.
Transparent electrodes 112 comprising a transparent electrically conductive film such as ITO are formed on the inner surface of the glass substrate 102, and an oriented film 113 is formed so as to cover the transparent electrodes 112. A ¼ wavelength plate 115 and a polarization plate 116 are provided on the outer surface side of the glass substrate 102.
In the liquid crystal display device 100, the surface of the glass substrates 101, 102 that is closer to the liquid crystal layer 103 is taken as an inner side and the surface that is farther from the liquid crystal layer is taken as an outer side.
The ¼ wavelength plates 115, 120 generate a phase difference of ¼ wavelength with respect to the independent polarization components in the light that are mutually orthogonal (this phase difference will be hereinbelow termed “λ/4 phase difference”), whereby the linearly polarized light and circularly polarized light are mutually converted.
In the liquid crystal display device 100, the light passing through the space sandwiched by the reflective display regions formed on the laminated members 151, 152 is used for the reflective display, and the light passing through the space sandwiched by the transmissive display regions formed at the two substrates is used for the transmissive display.
The liquid crystal display device 100 freely forms a bright-dark state in response to voltage application to the liquid crystal layer 103. Thus, in the liquid crystal display device 100, if a state is formed (ON state) in which a voltage is applied to the liquid crystal layer 103 and a state is formed in which practically no phase difference occurs in the light passing through the liquid crystal layer 103, then a dark state is formed, and if a state is formed (OFF state) in which no voltage is applied to the liquid crystal layer 103 and a state is formed in which a phase difference occurs in the light passing through the liquid crystal layer 103, then a bright state is formed.
The liquid crystal display device 100 forms a dark state in the reflective display in the manner as follows.
In the reflective display, the light (external light) from the outside of the polarization plate 114 falls toward the inside of the liquid crystal display device, but if the external light propagates via the polarization plate 114 inward the liquid crystal display device, then, of this light, the linearly polarized light (termed “incident linearly polarized light”) that is provided with a polarization axis parallel to the transmission axis of the polarization plate 114 is transmitted by the polarization plate 114 and propagates inwardly, and if this linearly polarized light further passes through the ¼ wavelength plate 120, then it becomes a circularly polarized light, and this circularly polarized light advances into the liquid crystal layer 103. Because no phase difference appears in the circularly polarized light when it passes through the liquid crystal layer 103, the circularly polarized light is transmitted via the liquid crystal layer 103, while maintaining the polarization state thereof.
The circularly polarized light that was transmitted via the liquid crystal layer 103 is reflected by the surface of the reflecting plate 104. At this time, the rotation direction of the circularly polarized light is reversed and it becomes an inversely rotated circularly polarized light and passes outwardly toward the ¼ wavelength plate 120 through the liquid crystal layer 103, while maintaining the polarization state thereof. If the circularly polarized light is transmitted through the ¼ wavelength plate 120, it becomes a linearly polarized light with a polarization axis perpendicular to that of the incident linearly polarized light. Here, because the polarization plate 114 has a transmission axis parallel to the polarization axis of the incident linearly polarized light, the linearly polarized light with a polarization axis perpendicular to that of the incident linearly polarized light is absorbed by the polarization plate 114 and practically no such light propagates to the outside. As a result, a person viewing the display sees practically no light. In other words, the liquid crystal display device performs the dark display.
The liquid crystal display device 100 forms a dark state in the transmissive display in the manner as follows.
In the transmissive display, the light from the light irradiation unit is introduced into the liquid crystal display device, but if the incident light propagates via the polarization plate 116 inward the liquid crystal display device, then, of this incident light, the linearly polarized light that is provided with a polarization axis parallel to the transmission axis of the polarization plate 116 is transmitted by the polarization plate 116 and propagates inwardly toward the ¼ wavelength plate 115. If this linearly polarized light further passes through the ¼ wavelength plate 115, then it becomes a circularly polarized light. Here, the polarization plate 116 and polarization plate 114 are disposed according to a cross Nicol's configuration, and the linearly polarized light obtained after passing through the polarization plate 116 becomes the linearly polarized light that has a polarization axis perpendicular to the incident linearly polarized light. Therefore, the circularly polarized light formed when the light passes through the ¼ wavelength plate 115 has the same rotation direction as the circularly polarized light formed by reflection at the surface of the reflecting plate 104 in the reflective display.
Therefore, the circularly polarized light formed by passing through the ¼ wavelength plate 115 propagates from the light transmitting unit via the liquid crystal layer 103 outwardly toward the ¼ wavelength 120. In the course of this propagation, the polarization state of light changes in the same manner as was explained with reference to the reflective display and eventually the light is absorbed by the polarization plate 114 and practically no light propagates to the outside. A person viewing the display sees practically no light from the light irradiation unit. In other words, the liquid crystal display device performs the dark display.
The liquid crystal display device 100 forms a bright state in the reflective display in the manner as follows.
From the introduction of the external light from the outside to the propagation to the liquid crystal layer, the light is transmitted, becomes a circularly polarized light, and propagates to the liquid crystal layer 103 in the same manner as in the formation of the dark state in the reflective display. With respect to the portions of the liquid crystal layer 103 through which the light is transmitted during the reflective display, settings for the liquid crystal layer 103 are made in advance such that when no voltage is applied to the liquid crystal layer, a ¼ wavelength phase difference is generated with respect to the light transmitted through the liquid crystal layer. Therefore, when the circularly polarized light propagating to the liquid crystal layer 103 is transmitted through the liquid crystal layer 103, the light becomes a linearly polarized light. Therefore, even if this light is reflected by the reflecting plate 104, it becomes a linearly polarized light with a polarization axis identical to that of this linearly polarized light. Furthermore, the linearly polarized light formed by reflection at the reflecting plate 104, propagates through the liquid crystal layer 103 outwardly toward the ¼ wavelength plate 120, is transmitted via the ¼ wavelength plate 120, and becomes a linearly polarized light having a polarization axis parallel to the transmission axis of the polarization plate 114. Further, this linearly polarized light passes through the polarization plate 114 and propagates outwardly. A person viewing the display sees the light, and the liquid crystal display device performs the bright display.
The liquid crystal display device 100 forms a bright state in the transmissive display in the manner as follows.
From the introduction of the light from the light irradiation unit to the propagation to the liquid crystal layer, the light is transmitted, becomes a circularly polarized light, and propagates to the liquid crystal layer 103 in the same manner as in the formation of the dark state in the transmissive display. With respect to the portion of the liquid crystal layer 103 through which the light is transmitted during the transmissive display, settings for the liquid crystal layer 103 are made in advance such that when no voltage is applied to the liquid crystal layer, a ½ wavelength phase difference is generated with respect to the light transmitted through the liquid crystal layer. Therefore, when the circularly polarized light propagating to the liquid crystal layer 103 is transmitted through the liquid crystal layer 103, the light becomes a circularly polarized light with reversed direction of rotation, and when this light is further transmitted through the ¼ wavelength plate 120, the light becomes a linearly polarized light having a polarization axis parallel to the transmission axis of the polarization plate 114. This linearly polarized light passes through the polarization plate 114 and propagates to the outside. Thus, a person viewing the display sees the light, and the liquid crystal display device performs the bright display.
In such liquid crystal display device 100, of the light (circularly polarized light) that is irradiated from the light irradiation unit and transmitted through the ¼ wavelength plate 115, the light that is reflected by the reflecting plate 104 and propagates outwardly toward the ¼ wavelength plate 115 is a circularly polarized light that has a rotation direction inverted with respect to that of the circularly polarized light directed inward of the liquid crystal display device, and if this light is transmitted outwardly via the ¼ wavelength plate 115, it becomes a linearly polarized light with a polarization axis perpendicular to the transmission axis of the polarization plate 116. Thus, eventually in the liquid crystal display device 100, the light reflected by the reflecting plate 4 is almost completely absorbed by the polarization plate 116, and the light from the light irradiation unit cannot be used effectively. The resultant problem is that the light utilization efficiency is decreased.
Accordingly, Japanese Patent Application Laid-open No. 2004-004494 suggested a semi-transmissive reflective liquid crystal display device in which a liquid crystal layer is sandwiched between mutually opposing upper substrate and lower substrate and a transmissive display region and a reflective display region are contained within one dot region, wherein an upper polarization plate is provided on the outer surface side of the upper substrate, a lower polarization plate is provided on the outer surface side of the lower substrate, a reflecting layer and a phase different layer are provided successively from the substrate side on the reflective display region on the inner surface side of the lower substrate, and when a selected voltage is applied or when a non-selected voltage is applied, the phase difference of the liquid crystal layer in the transmissive display region is larger than the phase difference of the liquid crystal layer in the reflective display region.
In this liquid crystal display device, laminating a phase difference layer that causes a ¼ wavelength (λ/4) only on the reflecting layer makes it possible to use the light efficiently and to improve the performance of the transmissive display.
However, the liquid crystal display device of Japanese Patent Application Laid-open No. 2004-004494 is manufactured by a process comprising the steps of using a photolithography method for patterning the phase difference layer on the substrate surface, laminating a photosensitive resin layer on a polymer liquid crystal layer, patterning the photosensitive resin layer, etching the polymer liquid crystal layer by using the patterned photosensitive resin layer as a mask, and leaving locally the polymer liquid crystal layer. Thus, the steps of forming and etching the photosensitive resin layer are required in the manufacture of the liquid crystal display device and the manufacturing process is complex.
At object of the present invention is to resolve the above-described problems and to provide an optical element that can be easily manufactured and can cause a phase difference in a desired position and also a method for manufacturing the optical element.
Another object of the present invention is to provide a semi-transmissive semi-reflective liquid crystal display device incorporating such optical element.